1. Field of the Invention
The present invention relates to a cutting tool with a cutting edge provided with a wear-resistant coating.
2. Background Art
Problems in forming a hole in a carbon fiber reinforced resin composite material using a cutting tool such as a drill bit are delamination, fiber raveling, and occurrence of burr, and the like.
As drill bits which are less likely to cause such problems, double angle drill bits described in Japanese Patent Unexamined Utility Model Publication No. 6-075612 and Japanese Patent Laid-open Publication No. 2008-036759 have been conventionally used.
The tip of a double angle drill bit described in Japanese Patent Unexamined Utility Model Publication No. 6-075612 is coated with diamond in order to increase in resistance to wear.
Herein, a description is given of a conventional typical drill bit provided with wear-resistant coating.
A drill bit shown in FIG. 10A is a typical example of conventional drill bits. In the drill bit shown in FIG. 10A, two grooves 103 are formed between a tip part 101 and a shank part 102. FIG. 10A shows helical grooves by way of example as the grooves 103. In the tip portion 101, two pairs of cutting edges 107 and 107 are formed. On one side of each cutting edge 107, a rake face 106 is formed, and on the other side thereof, a flank face 110 is formed.
The tip part 101 is cross-thinned, and thinnings 111 are continuous to the grooves 103. In each portion hollowed by the thinnings 111 and grooves 103, the rake face 106 is formed. The rake face 106 and flank face 110 meet at the top of the cutting edge 107 with an acute angle therebetween. Each pair of the cutting edges 107 is formed to have a point angle constant between a tool tip point O and a cutting edge maximum radium position RX.
Margins 105 are extended from the cutting edge maximum radium position RX rearward in the direction of an axis AX. In other words, a top ridge line 116A (see FIG. 10C) of each cutting edge 107 is connected to one of the margins 105 at the cutting edge maximum radius position RX. The top ridge line 116A of the cutting edge 17 and the margin 105 are not smoothly continuous to each other and have a relative angle at the cutting edge maximum radius position RX. The margins 105 come to contact with the inner surface of a drilled hole to support the drill bit and are formed in parallel to the inner surface of the drilled hole. On the other hand, since the cutting edges 107 are formed with a point angle constant from the tool tip point O, the angles of the cutting edge 107 and the margin 105 do not match at the cutting edge maximum radius position RX and are discontinuous.
As shown in the cross-sectional views of FIGS. 11A, 11B, 11C, and 11D, it is assumed that the drill bit is provided with a wear-resistant coating 112 from the cutting edge 107 to a margin formed portion where the margin 105 is formed.
In an initial state where the drill bit is not yet used in cutting, the top ridge line 116A of the cutting edge 107 is formed as indicated by solid lines in FIGS. 10B and 10C and is not worn. The states of the cutting edge 107 and wear-resistant coating 112 are shown in FIGS. 11A, 11B, 11C, and 11D.
If this drill bit is used for cutting, the wear-resistant coating 112 is removed by wear due to friction between the drill bit and a work W as shown in FIGS. 11E, 11F, 11G, and 11H. In the cutting edge 107 worn to a certain degree, the base material is exposed as shown in FIGS. 11E and 11F, and the exposed base material has begun to wear. The wear further proceeds. The larger the radius, the higher the speed relative to the work W, and the higher the cutting load. Accordingly, the drill bit is worn more in a cross section A1-A1 which is located at the comparatively backward position shown in FIG. 11F than in a cross section A-A which is located at the comparatively forward position shown in FIG. 11E.
The radius of the drill bit is large also in the margin formed portion shown in FIGS. 11G and 11H. However, the margin 15 does not perform cutting. Moreover, the area of contact between the margin formed portion and the work W is larger by the width of the margin 105, and the frictional load per unit area is smaller. The wear in the margin formed portion therefore does not proceed as much as in the cutting edge 107.
As a result, the cutting edge 107 which has been worn to a certain degree, as shown by a chain double-dashed line in FIGS. 10B and 10C, is hollowed near the cutting edge maximum radium position RX, which is the boundary between the cutting edge 107 and the margin 105, to have the cutting edge top ridge line 116B inflected. In FIG. 10C, Ho indicates an auxiliary line which passes through the cutting edge maximum radius position RX and extends in parallel to a tool central axis AX. Ha indicates an extension line of a cutting edge top ridge line 116A at the cutting edge maximum radius position RX in the initial time. Hb indicates an extension line of a cutting edge top ridge line 116B at the cutting edge maximum radius position RX when the cutting edge 107 is worn to a certain degree. As shown by the extension line Hb, the cutting edge point angle is close to 180 degrees at the cutting edge maximum radius position RX. With the cutting edges having a point angle of 180 degrees, the thinned margin at the outer circumference adversely affects accuracies of the hole diameter and the inner surface of the hole. Accordingly, at a certain time when the cutting edge point angle at the cutting edge maximum radius position RX is approximated to 180 degrees, the cutting performance of the drill bit is significantly reduced, and the drill bit comes to the end of the working life as a cutting tool.
This problem cannot be resolved even with a double angle drill bit having different cutting edge point angles. Moreover, even if the top ridge line of the cutting edge is connected smoothly to the margin, the difference in wear volume between the cutting edge and the margin formed portion does not change. Accordingly, the aforementioned problem cannot be resolved.
The studies by the inventors have confirmed that if the relief angle of the cutting edge is reduced, progress of wear is reduced because the area of contact between the cutting edge and the work W is increased to reduce the frictional load per unit area in a similar manner to the aforementioned margin formed portion. Accordingly, the wear volume of the cutting edge can be close to that of the margin formed portion. However, the following problem occurs. As shown in FIGS. 12A to 12D, if the relief angle of the flank face 210 of the cutting edge 107 is reduced (by about 10 degrees), wear proceeds in the order of FIG. 12A, FIG. 12B, FIG. 12C, and FIG. 12D. With the progress of wear, the area of a contact surface S1 between a part of the wear-resistant coating 212 on the flank face 210 and the work W increases. On the other hand, the area of contact between a part of the wear-resistant coating 212 on the rake face 206 and the work W is always equal to the cross-sectional area of the wear-resistant coating 212 in the layer thickness direction. Accordingly, wear of the wear-resistant coating 212 on the rake face 206 and the base material exposed at a cutting edge tip end 215 remain progressing while the progress of wear of the wear-resistant coating 212 on the flank face 210 is reduced. Accordingly, increasing R at the cutting edge tip end 215 reduces the cutting performance, thus causing the drill bit to come to the end of the working life as a cutting tool.